Between the solid crystalline phase and the fluid melt, intermediate phases occur in certain substances which combine structural and dynamic properties of both the ordered crystalline state and of the unordered melt state. Although these phases are fluid, they have, for example, optical properties which are characteristic of the majority of crystalline, but also partially crystalline substances. They are, for example, birefringent or have dielectric anisotropy. These are referred to as intermediate phases (mesophases) or alternatively liquid-crystalline phases. These phases can be obtained by varying the temperature, i.e., thermotropic liquid crystals, or in solution by varying the concentrations. Hereinafter, we are only concerned with thermotropic liquid crystals.
The existence ranges of these intermediate phases are generally characterized by, for example, transition temperatures, determined calorimetrically or by means of a polarizing microscope, from the crystalline state to the liquid-crystalline state (glass transition temperature) and from the liquid-crystalline state to the isotropic melt (clearing point) (cf. G. Allen & J. C. Bevington, Eds., Comprehensive Polymer Science, Vol. 5, pp. 701-732, Pergamon Press, 1989). If different liquid-crystalline states are present, the set of corresponding transition temperatures is indicated.
The structure of the liquid-crystalline phases is characterized by a different long-range and short-range degree of ordering of the molecules. A distinction is made between nematic phases, smectic phases and cholesteric phases. Cholesteric phases are also known as chiral nematic phases or twisted nematic phases.
In the nematic phase, the molecular centers are distributed without order, while the long axes of the molecules are aligned parallel to one another. This is different to the state in the fluid melt, where the molecule long axes are arranged randomly.
In the smectic phases, a regular arrangement of the molecular centers in space occurs in addition to the alignment order of the nematic phase described above. This regular arrangement can be present along one, but also along two or even three, independent spatial axes. These phases are nevertheless fluid. (Cf. Ullmanns Encyclopadie der Technischen Chemie, ed. 4, vol. 11, 657-671, Verlag Chemie 1976; H. F. Mark et al. Encyclopedia of Polymer Science and Engineering, 2nd ed., vol. 9, 1-61, J. Wiley & Sons 1987).
In the cholesteric phase, layers of nematically arranged molecules are arranged one on top of the other so that a continuous helical variation of the alignment direction of the molecule long axes is produced. The molecules thus form a helical structure with the period p. The cholesteric phase thus has a helical structure. Of the liquid-crystalline phases, it has particular properties (cf., for example, Bergmann-Schaefer, Experimental-physik, Vol. III: Optik, 7th Edition (1987), pp. 560-567, or de Vries, Acta crystallogr., (1951), 4, 219-226).
Thus, it is known that the cholesteric phase of liquid-crystalline substances in a macroscopic alignment in which the helical axes are arranged parallel to one another and perpendicular to the surface reflects light incident parallel to the helical axis (angle of incidence 0.degree.) in a wavelength range .gamma..sub.ref, which is determined by the period p, also known as the pitch, the refractive indices and the birefringence of the helical material (de Vries, H. I., (1951), Acta crystallogr., 4, 219; Meier, G. in: Physical Properties of Liquid Crystals, ed.: Meier, Sackmann, Grabmeier, Springer-Verlag, (1975), 9-11).
The reflected light is circular-polarized, the direction of rotation of the reflected light corresponding to the direction of rotation of the helical structure of the cholesteric phase (Jacobs, S. D., J. Fusion Energy, (186), 5(1), 65). If the helix axis is tilted by the angel .alpha., the reflection wavelength shifts in accordance with the Bragg law to .gamma..sub.ref x cos (.alpha.) (Emerle, H. J., Miller, A., Kreuzer, F.-H., Liquid Crystals, (1989), 5 (3), 907).
Other known liquid-crystalline phases having a helical structure are, for example, the S.sub.A * and S.sub.c * phases. In the S.sub.A * phase, the molecules within the layers are in an arrangement which is analogous to the S.sub.A phase. From layer to layer, however, the molecules are twisted with respect to one another. A twisting of the smectic layers is thus obtained (J. W. Goodby et al., A New Molecular Ordering in Helical Liquid Crystals, J. Am. Chem. Soc. (1989) 111, 8119-8125; T. J. Bunning et al., Bilayer structures in cholesteric, cyclic-siloxane liquid crystals, LIQUID CRYSTALS (1991), 10(4), 445-456). In the S.sub.c * phase, which is used, for example, in ferroelectric liquid-crystal displays, the tilt angle of the molecules forms a helical structure. For a suitable periodicity of the helix, selective reflection of light also occurs in this phase. These phases have the same optical properties as the cholesteric phase, which was described above.
An important property of liquid crystals is their birefringence. This is determined by the mean direction of the long molecular axes of the liquid-crystalline substance. The mean direction of said long molecular axes defines the so-called director (D. Demus, L. Richter, Textures of Liquid crystals, Verlag Chemie, Weinheim, New York 1978).
If it is desired to exploit certain anisotropic properties of these materials, they have to be suitably aligned.
Hitherto the light-induced alignment of liquid-crystalline dye-containing substances was carried out by illumination with polarized light. For example, it is known from DE 3920420 to align liquid-crystalline polymers by illumination with linear-polarized light in such a way that, after illumination, the director of the molecules lies in the plane which is generated by the direction of illumination and the perpendicular to the plane of polarization of the illuminating light. Consequently, after illumination with linear-polarized light the long molecular axes lie in one plane. Birefringence always occurs in liquid-crystalline substances aligned in this way.
Polarized light is, moreover, expensive. It is obtained either by means of a laser which emits in a polarized manner, or the light from normal lamps has to be linear-polarized. In the second case, half of the light of the lamp is lost; in addition, a limitation is imposed on the lamp powers which can be used by the polarizers.
From DE-A-3920421 it is known to align liquid crystals by means of circular-polarized light in such a way that the illuminated substance reflects circular-polarized light. Birefringence also always occurs in the liquid crystals aligned in this way.
It is furthermore known to align liquid crystals by applying an electric field in such a way that no birefringence occurs perpendicularly to the surface.